Domestic Hot Water

Weather Data

DHW Systems ComparisonRed Calc Tool User Guide

What this tool can do for you

This tool allows you to calculate the annual cost of heating domestic hot
water. In addition, you may compare the annual costs of up to three systems
to a base system. Four water heating fuel types are available; electricity,
natural gas, LPG, and #2 oil. You can use this tool for any system for which
you have an energy factor (EF), including electric resistance and heat pump
systems, and various types of oil- and gas-fired systems. Note that if a heat
pump water heater is installed in a condition or semi-conditioned space, some
or all of the heat transferred to the water during the heating season will
need to be replaced by the heating system. This "heating penalty" and the
corresponding "cooling benefit" during the cooling season, are not accounted
for by this tool.

An interactive chart displays the comparison of water heating systems, reporting
the annual energy usage and annual fuel cost for each. The chart can be printed or
saved in various formats, making it easy to include it in your reports or
presentations.

Source: U.S. Department of Energy

Source: U.S. Department of Energy

Calculated values

Annual hot water usage.

Annual fuel used.

Annual fuel cost.

Annual energy savings compared to base system.

Annual cost savings compared to base system.

Annual fuel usage.

Tips

Clicking the label for any input or result will cause a
popup help box to appear. This help box includes the
allowed and normal values (for inputs).
Read more.

The daily how water usage for a household in North America is from
45 to 70 gallons (170 to 265 liters), the average being 60 gallons
(228 liters). Per person hot water usage, is from
8 to 24 gallons (30 to 90 liters), the average being 16 gallons (60 liters).

The units selected for the prices in the Fuel Prices table are used to
calculate the results in the Systems Comparison table. For example, if
under Nat Gas in the Fuel Prices table, "$/therm" is selected and $1.20 is
the price input, any natural gas system included in the Systems Comparison
table will reflect the unit price of $1.20/therm.

The units selected under "Savings" in the second to last column of the
Systems Comparison table also sets the units used/displayed for
the "Annual energy usage" (left vertical axis) in the chart below this
table.

In the System Comparisons table, enter the data for the base water
heating system you wish to use. You may then enter data for up to three
alternate water heating systems that will be compared with the fuel used and the
fuel cost of the base system. In the table, each of the alternate systems
will show annual savings in energy units and cost compared with the base
system.

Check "Show chart" to display the interactive chart. The chart values
change as tool inputs above it are changed. Please refer to the Interactive
Chart Use section below for more information.

This tool can be used to determine the savings from lowering the
water heater temperature setting. For example, first enter the
"Temperature rise" of the higher temperature setting, say 90°F
(140 hot - 50 cold). With the use of the "Systems Comparison" table,
find the "Fuel used" and the "Fuel $". Now lower the
"Temperature rise" to 70°F (120 hot - 50 cold)
and use the "Systems Comparison" table to find the "Fuel used" and
"Fuel $" for the lower water temperature. Subtract these
lower use values from the higher use values to find the fuel and
dollar savings for lowering the temperature.

Interactive Chart Use

This tool includes a powerful interactive chart that can be used to display
results; it may also be used for educational, sales,
and marketing purposes. For example, you can save a chart in one of four different
file formats, embed it into a customer report, or print it separately.

Tips for using the interactive chart:

Click/touch the labels "Energy usage" or "Fuel cost" in the
legend below the chart to turn the corresponding chart bars on or off.

Hover/touch any of the bars in the chart to make a box appear
that displays the numerical value of the bar.

Click/touch the menu icon in the upper right corner of the chart
to show the choices for printing the chart or downloading it as an
image file in PNG, JPEG, PDF, or SVG format. You are free to use the
downloaded image in any way, including reports,
presentations, websites, as long as the attribution for
"Residential Energy Dynamics, LLC" is included.

If you save the chart as an SVG file, you can change the
title or any other chart element with the use of third-party
software. Examples of third-party software include Inkscape
(a free version is available) and Adobe Illustrator.

Inputs and field measurements

In order to use this tool, you must enter the energy factor (EF) for each
water heating system. If a system you are interested in appraising has a
published energy factor and used one of the four fuel sources
included in the tool, you can analyze it. Using the appropriate energy factor
is very important because it can significantly affect the results.

The current and future minimum U.S. energy factors are listed in Tables 1 and 2,
below. The minimum required U.S. energy factors in Table 2 take effect on
April 16, 2015.

The best source for energy factors for individual water heaters is
the AHRI Directory of
Certified Product Performance (Please see the reference below). For existing
water heaters, you will have to estimate the energy factor if it is not
listed on the water heater label. The values in Table 3 are intended for guidance
if you are not able to find an energy factor for a specific unit.

Source: U.S. Department of Energy

Source: U.S. Department of Energy

Table 3. Suggested Energy Factors for Water Heating Systems

Water Heating System

Suggested Energy Factor (EF)

Conventional gas storage

0.60

High-efficiency gas storage

0.65

Condensing gas storage

0.86

Demand gas with no pilot

0.82

Oil-fired storage

0.55

Low-efficiency electric storage

0.90

High-efficiency electric storage

0.95

Electric heat pump storage

2.20

Solar with electric back-up

1.20

Source: ACEEE (See "Related external resources" below)

Related tools

DHW Average Daily Usage:
This tool calculates average daily hot water use based on the amount of
hot water used by various appliances and use incidents. Default usage values
are included.

DHW First Hour Rating:
This tool helps you determine the First Hour Rating for the purpose of
sizing a storage water heater. This method is based on guidance from the
Air-Conditioning, Heating and Refrigeration Institute (AHRI), the
industry standard for sizing storage water heaters.

DHW Instantaneous Sizing:
This "solve-all" tool is used for sizing an instantaneous (tankless) hot water heater.
It can also be used to determine the maximum hot water
flow rate for a given system and temperature rise.

DHW Volume per Use:
This tool determines the volume of hot water used given an amount of mixed
(hot + cold) water used and the temperatures of the hot, cold, and mixed water.
The amount of mixed water can be specified by volume directly, or from a flow
rate and duration.

Water Flow Rate:
This is a "solve-all" tool that relates the volume of water drawn, the
duration of the water draw, and the flow rate. The primary expected use
case is determining flow rate using a test container with known volume
and a stop watch.

Related external resources

Air-Conditioning, Heating, and Refrigeration Institute (AHRI).
Directory of Certified Product Performance:
www.ahridirectory.org/ahridirectory/pages/home.aspx
Comment: This online directory is probably the best
source for energy factors for new water heaters. It is not necessary
to fill in all the inputs at the AHRI website to do a useful search
of equipment.

American Council for an Energy-Efficient Economy (ACEEE).
Water Heating:
http://aceee.org/consumer/water-heating#storage
Comment: This web page includes updated information
about water heater types and sizing, life-cycle cost comparisons, sizing a new
system, and efficiency standards.

California Energy Commission, Consumer Energy Center. Water Heaters:
www.consumerenergycenter.org/home/appliances/waterheaters.html
Comment: This web page includes a very good overview
of water heater issues, including pros and cons of the different types,
purchasing the right water heater, understanding energy factor, and
saving water heater energy.